The present invention relates to an optical disc having a plurality of recording layers and a recording method of the optical disc.
FIG. 2 schematically illustrates the sectional structure of a prior-art multilayered optical disc and the principle of reading and writing information in each recording layer selectively. In this prior-art example, the recording medium has 5 recording layers in total (first recording layer 411, second recording layer 412, third recording layer 413, fourth recording layer 414 and fifth recording layer 415). In this 5-layered recording medium, for example, in order to access information recorded in the second recording layer 412, the position of an objective lens 30 is controlled to position an optical spot 32 on the second recording layer 412. At this time, convergent light 31 focused by the objective lens passes through the first semitransparent recording layer 411 on the way thereof but the beam diameter of the convergent light 31 on the first recording layer is sufficiently larger than that of the optical spot 32 on the second recording layer 412. Accordingly, recorded information in the first semitransparent recording layer 411 cannot be analyzed or resolved to be reproduced. Since the beam diameter of the convergent light on the first semitransparent recording layer 411 is larger, the light intensity thereof per unit area is relatively small and it is not apprehended that information recorded in the first recording layer 411 is destroyed upon recording. In this manner, it can be realized that information can be recorded in or reproduced from the second recording layer farther than the first recording layer without influence of the first recording layer.
Similarly, when information is recorded in or reproduced from the fifth recording layer 415, the position of the objective lens 30 is controlled to position the optical spot 32 on the fifth recording layer 415. The beam diameter on the layer adjacent to the target layer for recording/reproduction is given by the following expression (1):2×d×(NA/n)/(1−NA/n)^2)^(½)  (1)where d is interlayer spacing between layers, NA a numerical aperture of the objective lens, λ a wavelength of light and n a refractive index of a transparent interlayer. For example, when d is 8 μm and NA is 0.85, the beam diameter is about 10 μm and is 20 or more times larger in diameter and 400 or more times larger in area as compared with the diameter λ/NA=470 nm of the optical spot 32 on the target layer at the time that wavelength is 400 nm. In this manner, the conditions for performing recording/reproduction to the optical recording medium having a plurality of recording layers without influence of other layers are described in JP-A-5-101398 in detail.
In such an optical disc having a plurality of layers, there arises a problem that when information is recorded in a farther layer as viewed from the light incident side, the laser power reaching the farther layer is different due to difference of the effective transmittance of the nearer layer between the case where information is recorded in the farther layer through unrecorded area in the nearer layer and the case where information is recorded in the farther layer through recorded area in the nearer layer. This problem is schematically illustrated referring to FIGS. 5A and 5B. FIG. 5A shows an optical spot focused on an n-th recording layer (n-th layer) and FIG. 5B shows an optical beam passing through the recording layer (m-th layer) nearer than the n-th layer when the optical spot is focused on the n-th layer. Vertical lines in FIGS. 5A and 5B show recording tracks formed in the n- and m-th layers. Area 431 is an area where any information is not recorded (unrecorded area) and area 432 is an area where any information is recorded (recorded area). In case of Blu-ray disc, the track pitch is 0.32 μm and the optical beam incident on the m-th layer spreads over the range containing about 100 tracks though it depends on the distance from the n-th layer to the m-th layer and the spread optical beam passes through the m-th layer. The transmittances of the recorded area and the unrecorded area are different and accordingly even when the same optical beam passes through the areas, the amounts of optical beams passing through the recorded and unrecorded areas are different. That is, the effective transmittance of the m-th layer is changed depending on a ratio of the recorded area and the unrecorded area in the m-th layer.
In order to cope with this problem, JP-A-2003-109217 discloses that a recording medium is structured so that difference between the transmittances of unrecorded part and recorded part in the nearer layer is smaller than or equal to a predetermined value, so that information can be recorded in the farther layer with fixed recording power irrespective of the recording state in the nearer layer.
As described in JP-A-2003-109217, when optical design of the nearer layer (m-th layer) is performed, it is desirable that the transmittance is not changed in the unrecorded and recorded areas. However, usually, the transmittance error of about several to ten % occurs between the unrecorded area 431 and the recorded area 432 due to various factors containing scattering in manufacture of medium and error in design. Moreover, even if the transmittance of the nearer layer can be made equal, the reflection factor is different and accordingly the quality of the reproduced signal from the farther layer is sometimes changed due to influence of the reflected light from the nearer layer.
Accordingly, in an actual medium, there is some transmittance difference between the unrecorded area 431 and the recorded area 432 and the laser power reaching the n-th layer in the case where information is recorded in the n-th layer on the farther side through the unrecorded area 431 of the m-th layer on the nearer side is different from that in the case where information is recorded in the n-th layer on the farther side through the recorded area 432 of the m-th layer on the nearer side due to difference of the effective transmittance of the m-th layer as shown in FIGS. 5A and 5B. More exactly, the effective transmittance of the m-th layer on the nearer side at the time that the optical spot 321 is focused on the n-th layer on the farther side is not changed in a binary manner or digitally but is changed continuously in accordance with the ratio of the unrecorded area and the recorded area occupied by the optical beam 322 on the m-th layer on the nearer side.
An example of influence that this phenomenon affects the recording condition learning is now described. FIG. 3 is a graph showing the relation between recording power and jitter at the time that recording/reproduction is performed to L0 layer on the farther side in 2-layer recordable type optical disc in case where L1 layer on the nearer side is not recorded and in case where L1 layer is recorded. The limit equalizer used normally in the Bru-ray disc is applied as the signal processing method for evaluation of reproduced signal and signal is expressed as the magnitude of signal jitter value. In this measurement, when all area of L1 layer on the nearer side is not recorded, the optimum recording power, that is, the recording power having the minimum jitter is 7.2 mW and the jitter at this time is 6.7%. On the other hand, when all area of L1 layer on the nearer layer is recorded, the optimum recording power is 7.5 mW. In other words, the optimum recording power in case where L1 layer is recorded is shifted by about 7% to the higher power side as compared with the case where L1 layer is not recorded. If the optimum recording power 7.1 mW in case where the L1 layer is not recorded is used to make recording when L1 layer is recorded, jitter is 7.0% and is increased by 0.3% as compared with the optimum recording power is used.
This result means the following, for example. When the recording condition learning for L0 layer is performed, it is supposed that part which the laser beam on L1 layer passes through is unrecorded and the optimum recording power decided in this state is used to make recording to the whole L0 layer. By doing so, when the part which the laser beam on L1 layer passes through is unrecorded, the recording can be made unproblematically, although when the part is recorded, jitter of reproduced signal of data recorded in L0 layer is increased. That is, the effective recording power margin is reduced. Accordingly, in the method described in JP-A-203-109217, the recording power margin is small and therefore it is difficult to make recording over the whole farther layer with high reliability using predetermined recording power.
In order to avoid such a problem, in the prior arts including JP-A-2005-038584 and JPA-2004-327038, areas for optimum power control of plural layers are adapted not to overlap one another, so that learning is always made in the state that the nearer layer is always unrecorded. Moreover, JP-A-2008-192258 discloses that the recording power is learned in both of the case where other layers are recorded and the case where other layers are unrecorded to calculate an average therebetween, so that a problem of error in the recording power learning is avoided.